Hi Suzanne:
Your will love both Water Parks,
Typhoon Lagoon and
Blizzard Beach. My boys, ages 8 and 6, love both Water Parks. So while this question has two right answers, I'll give you my family's opinion.
My boys are thrill seekers, and they enjoyed Blizzard Beach a bit more for the thrill rides, especially Summit Plummet. I would poll your party, if they don't enjoy thrill rides as much, then Typhoon Lagoon may be the preferable Park (if you choose Typhoon Lagoon, then hit Crush 'n' Gusher). Both Parks are large enough so that thrill seekers and those looking to relax should like both Parks. But if thrills are the name of the game, then I would look to Blizzard Beach.
To beat the crowds, get to the Water Parks at opening. It will help you claim the best lounge chairs and get you on a few rides with minimal wait.
Enjoy your vacation.